Peripheral blood leukocytes (PBL) from normal donors stimulated with Conconavalin A (Con A) generate cytotoxic effector cells (EC) which lyse allogeneic PBL from sarcoma patients but not PBL from normal donors. These EC also lyse allogeneic Epstein-Bar virus (EBV)-transformed cell lines, but not T cells from the same donors. They also lyse Daudi cells, which do not express Class I but do express Class II MHC antigens. These findings raise the possibility that Con A activated EC are detecting unique antigens expressed by virus-transformed cells and found in cancer patients but not normal leukocytes. These antigens could be modified Class II MHC antigens.